Cycloolefins, in particular, cyclohexene compounds are valuable as intermediates of organic chemical industrial products, and are particularly useful as starting mate rials for polyamides and lysine.
A variety of processes for producing cycloolefins are known and, of these processes, partial hydrogenation of monocyclic aromatic hydrocarbons in the presence of a ruthenium catalyst is most commonly employed. For improving selectivity and yield, the kinds of catalyst components and of carriers, metallic salts to be employed as additives to reaction systems and the like have been studied, and a number of results have been reported.
For example, the following suggestions have been made for the reaction systems in which water and zinc coexist and by which cycloolefins can be produced at relatively high yields.
(1) A process for partially reducing a monocyclic aromatic hydrocarbon with hydrogen in the presence of water and at least one kind of zinc compound under a neutral or acidic condition, using a particulate catalyst which mainly comprises metallic ruthenium having an average crystallite size of from 30 to 200 .ANG. and which is supported on a carrier (JP-B-8-25919; The term "JP-B" as used herein means an "examined Japanese patent publication").
(2) A process for producing a cycloolefin by partially hydrogenating a monocyclic aromatic hydrocarbon in the presence of a ruthenium catalyst, in which at least one of a zinc oxide and a zinc hydroxide at an amount of not more than saturation solubility is present in the reaction system in a completely dissolved state (JP-B-5-12331).
(3) A process for partially reducing a monocyclic aromatic hydrocarbon with hydrogen in the presence of water, in which a reaction is conducted using hydrogenated catalyst particles mainly comprising metallic ruthenium having an average crystallite size of 200 .ANG. or less in the presence of at least one kind of a solid basic zinc under a neutral or acidic condition (JP-B-8-19012).
(4) A process for partially reducing a monocyclic aromatic hydrocarbon with hydrogen using a ruthenium catalyst, which is prepared by reducing a ruthenium compound containing zinc beforehand, in the presence of water and a water-soluble zinc compound under an acidic condition (JP-B-2-16736).
In the process of producing a cycloolefin by partially reducing a monocyclic aromatic hydrocarbon with hydrogen using a ruthenium catalyst in the presence of water and zinc sulfate, the effect of concentration of zinc sulfate in the aqueous phase on the results of the reaction has been already examined, and it is known that there exists the most suitable zinc sulfate concentration which gives the highest selectivity of cycloolefin (Applied Catalysis A: General, 89 (1992) 77-102).
When the process of producing a cycloolefin by partially hydrogenating a monocyclic aromatic hydrocarbon with hydrogen using a ruthenium catalyst is applied to industrial practice, the catalyst is preferably used for a long time period because the frequency of catalyst replacement that may obstruct efficient production is desired to be made as less as possible. In this case, however, it was found that the selectivity of cycloolefin varies with time. After commencing the reaction using a catalyst, the selectivity of cycloolefin is generally improved at the very initial stage of the reaction, but after which the selectivity gradually decreases. The decrease in selectivity of cycloolefin, i.e., the objective product, should be avoided as much as possible for the efficient production.
However, the literature given above fails to mention any means of solving the problem regarding the decrease in the selectivity of cycloolefin caused by the long-term use of the catalyst. Even a description about the change, particularly the decrease, in selectivity of cycloolefin which is caused by the long-term use of the catalyst cannot found therein.